The 2C-subtype of serotonin receptor (5HT2C) has been implicated in a number of human psychiatric and behavioral disorders, including major depressive disorder, dysthymia, obsessive-compulsive disease, anxiety, and schizophrenia. Transcripts encoding the 5HT2C receptor can be modified by up to five adenosine-to-inosine RNA editing events, a process responsible for the cell-specific expression of as many as 24 receptor isoforms which may represent a regulatory mechanism by which cells modulate their response to extracellular signals by altering the efficacy and specificity of receptor:G-protein interactions. More recent studies have identified alterations in 5HT2C expression in patients diagnosed with anxiety, schizophrenia and depression associated with suicide and in response to antidepressant and antipsychotic treatment. The long term objectives of the proposed research are to define the cellular mechanisms involved in the regulation of 5HT2C expression and signaling, as well as possible relationships between 5HT2C editing and neuropsychiatric disorders. In re- cent studies, we have demonstrated a disparity between 5HT2C mRNA and protein isoforms as genetically modified mice solely expressing the fully edited isoform of the 5HT2C receptor exhibit an unprecedented 40- to 70-fold increase in 5HT2C receptor density compared to wild-type animals, yet 5HT2C mRNA levels remain un- changed. Thus, RNA editing has dramatic consequences on the expression of 5HT2C protein through uncharacterized post-transcriptional mechanism(s). The objectives of this proposal are to develop novel transgenic tools to investigate numerous aspects of 5HT2C receptor expression/function by the generation of embryonic stem cells harboring a Cassette Acceptor (CA) allele in which mutations may be introduced using recombinase mediated cassette exchange, a process that can rapidly and efficiently insert different DNA fragments into specific gene loci and is significantly more efficient than homologous recombination. Here we focus upon the introduction of epitope tag(s) into the endogenous 5HT2C locus, allowing a more effective purification of 5HT2C receptor protein for subsequent comparisons of mRNA and protein isoform distribution in discrete brain regions. The functional consequences of epitope insertion at multiple sites within the receptor will be assessed in transfected heterologous cell lines by examining potential alterations in receptor expression, ligand affinity and signaling. The insertion of an epitope that does not alter receptor function will be introduced into mice and mutant animals will be assessed for alterations in behavior, receptor expression/function and the ability to purify the tagged receptor using tandem affinity chromatography. It is anticipated that the development of this novel transgenic strategy will not only provide tools to examine potential disparities in 5HT2C mRNA and protein expression, but also provide numerous researchers with a more efficient method to introduce any mutation of interest (e.g. disease-associated SNPs, reporter constructs, toxins, null/conditional alleles) into the 5HT2C receptor gene, thus expanding research into human psychiatric disorders related to altered 5HT2C function.